1. Field of the Invention
The present invention relates to a motor control device and a motor control method.
2. Discussion of the Related Art
Heretofore, a motor control device for controlling the operation of a brushless motor is provided with an output circuit composed of a plurality of switching elements such as power MOSFETs or the like and supplies the brushless motor with drive electric powers for three phases U, V and W generated from a direct current, by controlling duty operations of the switching elements.
In the motor control device of this type, there has been in wide use a triangular wave comparison PWM (Pulse Width Modulation) for generating motor control signals for the U, V and W phases by comparing a triangular wave signal (hereafter referred simply as “triangular wave”) as a carrier wave with phase voltage commands Vu*, Vv* and Vw* which respectively take as reference waves sine waves represented by the following expression (1).
                    (                  Expression          ⁢                                          ⁢          1                )                                                                                                                                  Vu                  *                                ⁢                                                                  =                                                      α                    ·                    Vc                    ·                    sin                                    ⁢                                                                          ⁢                  θ                                                                                                                          Vv                  *                                ⁢                                                                  =                                                      α                    ·                    Vc                    ·                    sin                                    ⁢                                                                          ⁢                                      (                                          θ                      -                                              2                        ⁢                                                                                                  ⁢                                                  π                          /                          3                                                                                      )                                                                                                                                            Vw                  *                                ⁢                                                                  =                                                      α                    ·                    Vc                    ·                    sin                                    ⁢                                                                          ⁢                                      (                                          θ                      +                                              2                        ⁢                                                  π                          /                          3                                                                                      )                                                                                      }                            (        1        )            
α: (−1≦α≦1): modulation factor (control factor)
Vc: amplitude of triangular wave
In the conventional triangular wave comparison PWM, where the supply voltage is taken as VB, sine waves are formed by the U, V, W phase voltages at respective output points relative to an imaginary neutral point in the direct current power supply, that is, by the reference waves of respective phase voltages VuN, VvN, VwN, and the amplitude of the sine waves becomes α·VB/2. The amplitude of the line voltages Vuv, Vvw, Vwu between the U, V, W phases becomes square root of three (√3) times as large as the maximum amplitude of the reference waves of the respective phase voltages VuN, VvN, VwN. That is, the amplitude of the line voltages Vuv, Vvw, Vwu becomes α·√3/2·VB.
By the way, in brushless motors, a problem arises in that back electromotive force which is exerted on the coils upon the rotation of an armature becomes greater than the line voltages Vuv, Vvw, Vwu between the respective phases with increase in the rotational speed, so that the electric current can no longer be flown through the coils. Therefore, in order to operate the motor in a high speed, it is necessary to increase an inverter output voltage so that the line voltages Vuv, Vvw, Vwu between the U, V, W phases to be supplied to the brushless motor can be raised (i.e., heightened).
However, in the known PWM with triangular wave comparison, the amplitude of the phase voltage commands Vu*, Vv*, Vw* has to be smaller than that of the triangular wave. For this reason, in a conventional PWM with triangular wave comparison, if the maximum value of the amplitude of the reference waves of the line voltages Vuv, Vvw, Vwu between the respective phases exceeds √3/2·VB, the proportional relationship is lost between the amplitude of the reference waves of the line voltages Vuv, Vvw, Vwu for the respective phases and the modulation factor (α). This gives rise to a problem that the PWM control cannot be performed normally thereby causing the brushless motor to generate noise and torque ripple.
To obviate this problem, if an attempt is made to make the maximum value of the amplitude of the reference waves of the line voltages Vuv, Vvw, Vwu greater than √3/2·VB, the supply voltage VB itself has to be heightened by using a booster circuit or the like. In the case so attempted, there arises another problem that besides the addition of the booster circuit, review is required regarding the withstand voltage capability of the output circuit including switching elements thereby resulting in substantial increase in cost.
As a method for solving the foregoing problems, there has been known one which is described in a literature entitled “Theory and Practical Design for AC Servo System” by H. Sugiyama et al., Ver. 4 Feb. 10, 1997, pp. 44–47, Sougou-Denshi Publishing Company, Tokyo, Japan. In the method described in the literature, the rate of an inverter output voltage to a supply voltage VB, that is, the utilization factor of a power supply is improved by superposing a third-order harmonic wave on each of the reference waves of the phase voltage commands Vu0*, Vv0*, Vw0* and by generating motor control signals based on the superposed phase voltage commands Vu1*, Vv1*, Vw1*.
In the known method, the amplitude of the reference waves of the phase voltage commands Vu0*, Vv0*, Vw0* is raised to 2/√3·α·Vc, and the motor control signals are generated based on the phase voltage commands Vu1*, Vv1*, Vw1* each of which has the third-order harmonic wave superposed thereon to have reference waves represented by the following expression (2), as shown in FIG. 6.
                    (                  Expression          ⁢                                          ⁢          2                )                                                                                                                                  Vu1                  *                                ⁢                                                                  =                                  2                  ⁢                                      /                                    ⁢                                                            √                      3                                        ·                    α                    ·                    Vc                                    ⁢                                                                          ⁢                                      (                                                                  sin                        ⁢                                                                                                  ⁢                        θ                                            +                                              1                        ⁢                                                  /                                                ⁢                                                  6                          ·                          sin                                                ⁢                                                                                                  ⁢                        3                        ⁢                                                                                                  ⁢                        θ                                                              )                                                                                                                                            Vv1                  *                                ⁢                                                                  =                                  2                  ⁢                                      /                                    ⁢                                                            √                      3                                        ·                    α                    ·                    Vc                                    ⁢                                                                          ⁢                                      (                                                                  sin                        ⁡                                                  (                                                      θ                            -                                                          2                              ⁢                                                                                                                          ⁢                              π                              ⁢                                                              /                                                            ⁢                              3                                                                                )                                                                    +                                              1                        ⁢                                                  /                                                ⁢                                                  6                          ·                          sin                                                ⁢                                                                                                  ⁢                        3                        ⁢                        θ                                                              )                                                                                                                                            Vw1                  *                                ⁢                                                                  =                                  2                  ⁢                                      /                                    ⁢                                                            √                      3                                        ·                    α                    ·                    Vc                                    ⁢                                                                          ⁢                                      (                                                                  sin                        ⁡                                                  (                                                      θ                            +                                                          2                              ⁢                              π                              ⁢                                                              /                                                            ⁢                              3                                                                                )                                                                    +                                              1                        ⁢                                                  /                                                ⁢                                                  6                          ·                          sin                                                ⁢                                                                                                  ⁢                        3                        ⁢                        θ                                                              )                                                                                      }                            (        2        )            
In this method, even where the amplitude of the reference waves of the phase voltage commands Vu0*, Vv0*, Vw0* is raised to 2√3·α·Vc, the maximum value of the phase voltage commands Vu1*, Vv1*, Vw1* after the superposition becomes |α·Vc| and does not exceed the amplitude Vc of the triangular wave, because the maximum value (absolute value) of the amplitude of (sin θ+1/6·sin 3θ) is √3/2.
Accordingly, it is possible to perform the PWM control normally within the range of −1≦α≦1 without loosing the proportional relationship between the amplitude of the reference waves of the line voltages Vuv, Vvw, Vwu for the respective phases and the modulation factor (α). As a consequence, it can be realized to raise up to the supply voltage VB the maximum value (absolute value) of the amplitude of the line voltages Vuv, Vvw, Vwu for the respective phases. That is, by employing the third-order harmonic wave superposition method as described above, it can be realized to heighten the inverter output voltage without boosting the supply voltage VB and hence, to raise the line voltages Vuv, Vvw, Vwu for the respective phases U, V, W to be supplied to the brushless motor.
However, for the employment of the foregoing third-order harmonic wave superposition method, it is required to provide a table which has stored arithmetic results of sin 3θ in advance or to calculate the sin 3θ by arithmetic operation one after another. To this end, where the table is used, it is unavoidable that the manufacturing cost increases with increase in the memory capacity for storing the table. Instead, where the sin 3θ is calculated by arithmetic operation, there arises another problem that the performance in control is liable to be degraded due to substantial delay in operation interval caused by increase in the operation load.